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This article written by Nikki Levin, MD, PhD
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This article is part of the Nikki Levin Recovery Project
Introduction: what is athlete’s foot? You don’t have to be an athlete to get athlete’s foot. In fact, anyone with sweaty feet can acquire this common superficial fungal infection. Athlete’s foot is a rash that occurs on the soles of the feet and the skin between the toes (Figure 1). It is the most common fungal infection in the United States and is estimated to affect up to 70% of the world’s population at some time in their life. The medical term for athlete’s foot is tinea pedis.
Figure 1. A typical case of athlete’s foot, showing scaling on the bottom of the foot and between the toes. Photograph courtesy of Dr. Amit Garg.
What causes athlete’s foot?
Athlete’s foot is caused by fungi known as dermatophytes, a term derived from the Greek, meaning “skin plants.” Dermatophytes are filamentous fungi that grow on humans, animals, or in the soil. These fungi live on keratin, a protein found in the outermost layer of human skin, as well as in hair and nails. The fungi are not able to penetrate beyond the outer layer of the skin or cause internal disease.
The fungal species that commonly cause athlete’s foot are Trichophyton rubrum, Trichophyton mentagrophytes, Trichophyton tonsurans (in the rare cases that occur in children), and Epidermophyton floccosum. Less commonly, Scytalidium molds and Candida yeasts may cause athlete’s foot. The relative frequency of these fungal pathogens varies from one part of the world to another. For example, whereas T. rubrum caused 63.3% of cases of athlete’s foot in a Japanese survey, it was responsible for only 17% of cases in Algeria, where Candida yeasts caused 20% of cases.
Who gets athlete’s foot?
Athlete’s foot is common worldwide in adults of both sexes and all races. It is uncommon in children, occurring in only 0.15% of children aged six-14 in a Turkish study. A small study comparing professional and college soccer players to non-athletes did show that athlete’s foot was significantly more common in the soccer players, but this condition is by no means exclusive to athletes. It is also common in military personnel, boarding school students, and farm workers.
Athlete’s foot is a contagious disease. It may be acquired by using locker rooms and communal showers that are contaminated with the causative fungi. It is more common in areas of high heat and humidity and in people who wear shoes. In fact, athlete’s foot is quite rare in cultures where people go barefoot. The fungi that cause athlete’s foot require moisture in order to grow, so sweaty feet that spend 12 hours a day shut inside shoes provide the perfect environment.
The fungi that cause athlete’s foot also may cause jock itch (tinea cruris), toenail infection (onychomycosis), and fungal infection of the hand (tinea manuum). Having any of these other fungal infections is a risk factor for athlete’s foot. In a study of 1181 patients in Poland with toenail fungus, 33.8% also had athlete’s foot. Two thirds of these had the interdigital type of athlete’s foot, which affects the skin between the toes. This is a higher frequency of athlete’s foot than would be expected in the general population.
Interestingly, some people seem to be much more susceptible to athlete’s foot than others, despite having an otherwise normal immune system. It is possible that subtle differences in a type of immunity known as the innate immune system may make some people more resistant to athlete’s foot. Studies have shown that a protein with antimicrobial properties, known as beta-defensin-2, is elevated in skin from people with athlete’s foot, suggesting that this protein may play a role in host defense against fungus.
To summarize, risk factors for athlete’s foot include:
- living in a warm humid climate
- wearing air-tight shoes
- using locker rooms and public showers
- having another fungal infection such as jock itch, fungal toenails, or fungal infection of the hand
- diabetes mellitus
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